I've got about the same problem here. Several 5m strings of 300 wonderful 5630 LED lights. But the strips require 12V @ 5A. When I need a number of these strips, I either need to raise the voltage and chain them in series (no more than 48VDC, though, to stay under safety guidelines), or else I need to have a very beefy power supply.
Galvanic isolation with a transformer works nice. But have you ever priced one of those things for high currents??? You'll choose instead to learn how to wind them yourself from some retired microwave oven transformer core, pretty quickly.
Or else you'll need to explore isolated switching supplies. PC power supplies start looking nicer. An older 500W power supply might provide 30A on the +12V rail. But for you, wanting 5V, they supply even more -- perhaps 50A? It's still a big box. But it weighs less than some of those heavy transformers used in a linear DC supply. And they are nearly free, if you just look around for a moment.
Don't forget about current carrying capacity over copper wire and sizing that appropriately (which means taking into account longer runs, when applicable.) You should size for twice what the wire carries given the length, as well. If you plan a drop of no more than 0.25V across the wiring (summed on both directions), then at 9A that's still more than 2 watts within the wiring itself. Also, keep in mind that your LED strips will get hot. Luckily, that's over a long length so it is often not such a problem.
I use screw terminals, like these: https://www.amazon.com/dp/B0079G38L6 or these: https://www.amazon.com/dp/B00EZ3QPCU (the first one says 15A, the second one says 8A and is probably too small for your use.) Then I just use a heavy gauge wire and screw it down. But there are similar connector systems which are pluggable: https://www.amazon.com/dp/B00GWF65WY . But just be aware of their current ratings before you buy anything.
I'm assuming you aren't planning on controlling the ON/OFF of the power rail to the LEDs from an I/O pin on your raspberry pi. Right?
